(1) Field of the Invention
The present invention relates to a switching power supply device and a semiconductor device used for the same.
(2) Description of the Related Art
A buck converter is widely known as a step-down non-isolated switching power supply device which converts an input power supply voltage into a lower voltage and outputs the converted voltage.
FIG. 13 shows an example of a general buck converter switching power supply device. Since the detailed operation of the buck converter is familiar to one skilled in the art, the following description focuses on aspects relevant to embodiments of the present invention. In FIG. 13, the voltage regulation of a direct current (DC) output terminal 114 relative to a power supply reference terminal 113 is determined by an output voltage detection circuit which includes a zener diode 112 and a photocoupler 110 and by an output voltage regulation circuit included in an integrated circuit 101 connected to a terminal 103. In one configuration, the integrated circuit 101 may use a power supply controller in which a switching element and a circuit for controlling timing of the driving of the switching element are integrated. The output voltage detection circuit in FIG. 13 has a voltage threshold determined by the breakdown voltage of the zener diode 112 and the forward voltage of the photodiode 115 in the photocoupler 110.
In the configuration shown in FIG. 13, the internal circuit of the integrated circuit 101 connected to the terminal 103 detects a current flowing through a phototransistor 116 of the photocoupler 110 to control the switching of a semiconductor switch included in the integrated circuit 101. The control of this internal semiconductor switch controls the delivery of energy from the power supply input terminal 107 to the DC output terminal 114, and the output voltage of the output terminal 114 relative to the power supply reference terminal 113 is regulated.
A disadvantage of using the integrated circuit 101 as shown in FIG. 13 is that since the internal circuit connected to the terminal 103 detects a current flowing out of the terminal 103, a separate circuit configuration is required for creating a current flowing out of the photocoupler 110 and the terminal 103 as shown in FIG. 13. This results in increasing the overall circuit cost.
Patent Reference 1 (Japanese Patent Application Publication No. 2005-12993) discloses an example of an idea for reducing cost of such a general buck converter switching power supply device. Hereinafter, the conventional switching power supply device disclosed in Patent Reference 1 is described with reference to FIG. 14. FIG. 14 is a circuit diagram of the conventional switching power supply device.
As shown in FIG. 14, a conventional switching power supply device 300 is a circuit in which an input power supply voltage Vin is applied to a power supply input terminal 313 relative to a power supply reference terminal 314 and an output voltage V0 is output to an output terminal 312 relative to the power supply reference terminal 314. The switching power supply device 300 includes: a power supply controller 301; a coil 309 which serves as an energy conversion circuit; a diode 311; a capacitor 310; a diode 307 which detects output voltage to regulate the output voltage of the power supply; a smoothing capacitor 308; and a resistor 306.
The power supply controller 301 includes at least a switching element and a control circuit which detects a signal applied from outside to control the switching of the switching element. The power supply controller 301 also includes four terminals that are a drain terminal 305 connected to the drain of the switching element, a source terminal 302 connected to the source of the switching element, a bypass terminal 303, and a current detection terminal 304. The source terminal 302 is a voltage reference terminal of the power supply controller 301, and stores electric charge required for the power supply voltage of the power supply controller 301 in the bypass capacitor 315 connected between the source terminal 302 and the bypass terminal 303. The drain terminal 305 is connected to the power supply input terminal 313, and receives an input power supply voltage Vin. The source terminal 302 is connected to the diode 311, the coil 309, and one terminal of the smoothing capacitor 308. The current detection terminal 304 is connected to the resistor 306.
Since the forward voltage drop across the diode 307 compensates the forward voltage drop of the diode 311, the voltage Va across the smoothing capacitor 308 during the period that the diode 311 is conducting is substantially equal to the output voltage V0 relative to the voltage of the power supply reference terminal 314. Further, during the period that the switching element in the power supply controller 301 is ON, the voltage Va across the smoothing capacitor 308 is held by the reverse withstand voltage of the diode 307. Accordingly, the voltage Va at a positive terminal of the smoothing capacitor 308 does not have potential lower than that of the voltage reference terminal (source terminal 302) of the power supply controller 301.
Further, the positive terminal of the smoothing capacitor 308 has a voltage that is representative of the regulated output voltage of the power supply, that is, a voltage that is derived from the regulated output voltage of the power supply. In other words, the output voltage V0 relative to the power supply reference terminal 314 is rectified and smoothed by the diode 307 and the smoothing capacitor 308, and then converted into the voltage Va that appears at the positive terminal of the smoothing capacitor 308 relative to the power supply reference terminal (source terminal 302) of the power supply controller 301. As a result, the absolute values of Vo and Va are substantially equal to each other.
The current detection terminal 304 is connected to a current detection circuit included in the power supply controller 301. The current detection circuit included in the power supply controller 301 has a current threshold. The voltage of the current detection terminal 304 relative to the voltage reference terminal (source terminal 302) is substantially constant when a current equal to the current threshold is conducted in the current detection terminal 304.
The smoothed voltage Va relative to the voltage reference terminal (source terminal 302) of the power supply controller 301 is converted to a DC current smoothed by the resistor 306. Accordingly, the smoothed DC current is applied to the current detection terminal 304 of the power supply controller 301 as a DC detection current Iz which is representative of information of the output voltage V0. The current detection circuit included in the power supply controller 301 has a current threshold. The current detection circuit compares the detection current Ia applied from the current detection terminal with the current threshold, and, outputs a digital signal for turning off the switching element when the DC detection current Iz is above the current threshold.
An output from the current detection circuit included in the power supply controller 301 is used to control the switching of the switching element included in the power supply controller 301. The power supply controller 301 controls the transfer of energy from the power supply input terminal 313 to the output terminal 312 and hence regulates the output voltage V0 of the power supply between the output terminal 312 and the power supply reference terminal 314. The output voltage of the power supply can be regulated by choosing the value of the resistor 306 such that the voltage across the smoothing capacitor 308 reaches the current threshold.
A general buck convertor switching power supply device requires an expensive photocoupler and a zener diode as an output voltage detection circuit. Whereas, the output voltage detection circuit of the switching power supply device disclosed in Patent Reference 1 uses a diode and a smoothing capacitor instead. This allows reduction in cost of the output voltage detection circuit.